The joint session history of a top-level browsing context is the union
of all the session histories of all browsing contexts of all the fully active Document
objects that share that top-level browsing context, with all the entries that are
current entries in their respective session histories removed except for the current entry of the joint session
history.

When the go(delta) method is
invoked, if the argument to the method was omitted or has the value zero, the user agent must act
as if the location.reload() method was called instead. Otherwise, the user agent must traverse the history by a delta whose value is the
value of the method's argument.